Inorganic substrates have been coated with compositions that contain pigments to impart color properties to the substrate for aesthetic purposes. The coated substrates are generally applied or affixed to specific carriers to provide a desired color to the object. For example, coated inorganic granules are often utilized on roofing shingles.
Composite roofing shingles usually employ a web of fibrous base material. The base material typically is a nonwoven fabric such as felt, fiberglass, or similar materials, which are impregnated with a bituminous composition such as asphalt. A layer of bituminous material typically is also applied onto one side of the base material. Roofing granules are then pressed into the layer of bituminous material while it is in a warm, softened condition. Upon cooling, the roofing granules remain attached as a surface layer. The layer of roofing granules functions as a protective layer to shield the bituminous material and the base material from both solar (e.g. ultraviolet radiation) and environmental degradation. Additionally, the roofing granules are generally colored to provide an aesthetically pleasing appearance to the building or structure upon which the shingles are applied.
The coloring of the shingles is an important feature which renders a desired appearance to a structure. It is preferred that the color of the shingles resist fading and maintain the original color over time. Certain colors have had a greater tendency to fade thus yielding an undesirable effect in the color of the granular coated shingle. Black colors, specifically medium blacks, have a greater tendency to fade when exposed to weathering conditions, such as temperature cycles and acidic precipitation. Medium blacks, as defined with CIELAB color scale coordinates, generally have an L* value within the range of about 24 to 37. Black colored roofing granules are often blended with one or more colored granules of a different hue to provide a specific color blend. This blend, when viewed from a distance, appears as a uniform color. Any resulting color change due to fading can adversely affect the desired color of the shingle.
Inorganic granules having a black color are produced by coating an inorganic granular material with a slurry containing an inorganic binder and black pigment particles. The pigments are generally carbon black particles with a mean particle size of less than 110 nm. The inorganic granular material is heated in a rotary kiln and mixed with the slurry of inorganic binder and pigment particles. The coated inorganic granules are first dried and then fired at temperatures in excess of 170.degree. C. to insolubilize the binder. While each of the resulting coated granules exhibit a desirable black color, the color has a tendency to fade over time when exposed to weathering conditions.
Thus, it would be an advantage to provide a coating composition that is capable of imparting a fade resistant black color to an inorganic granular substrate.